A type of variable resistor device commonly referred to as a “memristor” is a passive two-terminal circuit element in which its resistive state depends upon the voltage or current history of the element. More specifically, the resistive state is characterized by the relationship between the charge and flux. The resistance may vary continuously, or it may change discontinuously between distinct, non-volatile resistance states in resistively switched devices. Memristors retain their resistive states even when no potential is applied across them. They provide opportunity for improved solutions for high-density and energy efficient non-volatile data storage.
Memristors can behave as a digital as well as analogdevice, and therefore, they are not only feasible for performing storage operations but they can also serve as components of analog computer architectures such as artificial neural networks (ANNs), which can out-perform von Neumann architectures in many computational tasks. The use of memristors in ANNs is desirable because such components may facilitate the conversion of software-based ANN implementations to entirely hardware-based platforms which are faster, more efficient and easier to manufacture. Nevertheless, the use of memristors in many practical implementations is prohibited because macroscale and high-power memristor devices have not been achievable.
Nanometer-scale memristive devices can be used in many integrated microelectronics applications, however, they have a low current density tolerance and thus their use in high-power system applications is limited. The most widely investigated and successfully demonstrated class of materials for memristive phenomena are metal-oxides. In memristive metal-oxide devices, ionic defects migrate under the influence of an applied electric field and vary in density throughout the volume of the metal oxide. Variable resistance arises when the defect mobility is high enough to cause defect density gradients across the metal-oxide structure within practical time parameters. Since there is a limited ion migration path length in memristive metal oxides, many of these oxides used for state of the art memristors do not exhibit the same properties at a macroscopic scale, and therefore, macroscale memristive devices have not been achievable. Thus, there is a need for memristive materials and devices for broader application.